Filled rubber compounds, flow

Flow Singularities of Filled Rubber Compounds A Brief Overview.820... 

It has long been reported in literature [18, 19] that (carbon black) filled compounds are yield stress materials, i.e., when plotted versus the shear stress, the shear viscosity appears bounded by a critical shear stress Oc so that below it, no flow occurs (in other words, the viscosity goes to infinity as the shear stress decreases towards Oc). The right graph in Fig. 4 shows indeed that the shear viscosity q(o) increases, as the shear stress decreases, but one would hardly derive a bounding critical shear stress from such data. In other terms, that filled rubber compounds are essentially nonlinear viscoelastic materials is experimentally well demonstrated but that they are yield stress materials might be considered as a controversial subject. 

Carbon black filled rubber compound morphology and nonlinear flow properties. 

Flow induced anisotropy effects in filled rubber compounds. (Drawn using data from K. Nakashima, H. Fukuta, M. Mineki, /. Appl. Polym. Sci., 17, 769-778,1973.)... 

Basic studies of flow mechanisms in molding have been lacking for rubber compounds. Almost all published studies have been for thermoplastics. Most of these investigations involve injection mold filling. 

This class of material is utilized by the rubber compounder when he has a problem concerned with improvement of material flow in such products as intricate mouldings manufactured by compression or more often by injection moulding. To be economic, injection moulding needs compounds which will fill the mould rapidly and which will have consistent flow characteristics to ensure that all cavities of multi-cavity moulds are filled to make good products. 

Although the material behaviour at the feed port can be observed, once the compound enters the screw channel, direct observation is not possible. The flow visualisation is accomplished for thermoplastics, by stopping the screw, cooling the machine and pushing out the screw with the material filling the channel. For a rubber compound the rotation... 

Materials which such a complex structure cannot of course exhibit a simple rheological behavior, and relatively easy arguments may be produced to explain—so far qualitatively—how this complex structure affects most of the flow singularities of rubber compound. As illustrated in Figure 5.17, typical nonlinear effects observed with CB filled compounds appear as logical consequences of such a soft three-dimensional network of complex rubber-aggregate entities with connective filaments. 

From about 1980, there have been extensive investigations of the shear viscosity of rubber-carbon black compounds and related filled polymer melts. Yield values in polystyrene-carbon black compounds in shear flow were found by Lobe and vhiite [L15] in 1979 and by Tanaka and White [Tl] in 1980 for polystyrene with calcium carbonate and titanium dioxide as well as carbon black. From 1982, White and coworkers found yield values in compounds containing butadiene-styrene copolymer [Ml, M37, S12, S18, T7, W29], polyiso-prene [M33, M37, S12, S18], polychloroprene [S18], and ethylene-propylene terpolymer [OlO, S18]. Typical shear viscosity-shear stress data for rubber-carbon black compounds are shown in Figs. 5(a) and (b). White et al. [S12, S18, W28] fit these data with both Eq. (56) and die expression... 

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